1. Field of the Invention
This invention relates in general to electrical cables for use in hostile environments, and in particular to an electrical cable for use in an oil and gas well to conduct electrical power to a downhole submersible pump.
2. Description of the Prior Art
This invention concerns electrical cables of the type which are used to power downhole electric motors for submersible pumps within oil and gas wells. These submersible pumps normally pump a mixture of oil and brine from wells often several thousand meters (feet) deep and often under high temperatures and pressures. The electrical cables normally consist of three stranded or solid conductors. Each stranded or solid conductor contains an insulating layer of a material that is resistant to oil and brine. Typically, in a round configuration, an elastomeric, protective jacket is extruded to cover all three conductors, and an outer metallic armor surrounds the jacket.
The elastomeric, protective jacket is typically formed from a thermally set elastomeric material and is used to provide a seal to prevent wellbore fluids from reaching a thermoplastic insulation which forms a sleeve about each of the electrical conductors within the electric cable. Prior art thermally set elastomeric protective jackets are cured either before or after adding the exterior armor by heating the jacket to an elevated temperature for a period of time sufficient to vulcanize the thermally set elastomeric material from which the protective jacket is formed. Please note that the term vulcanize, as used herein, does not necessarily indicate the use of sulfur to cure the elastomeric material.
The period of time required to fully cure a protective jacket about insulated conductors is determined by the combination of time: to first heat the entire elastomeric material to a cure temperature, and then, the length of time at which the elastomeric material must remain at that temperature to fully cure. Typically, the elastomeric material within the interstices between the insulated conductors is the last portion of the protective sleeve to be heated to the cure temperature, and consequently the last portion of elastomeric material to cure.
One method of curing the thermally set elastomeric material which provides the protective jacket is in a continuous cure process, such as a continuous vulcanization process. One typical prior art continuous vulcanization process included a vulcanization tube which was 91 meters (300 feet) long, of which about two-thirds of the length was filled with pressurized steam at a temperature of 204.degree. C. (400 degrees Fahrenheit) and a pressure of 1.7 megapascals (250 pounds per square inch). The thermally set elastomeric material was extruded about the insulated conductors to form a protective jacket, and then passed through the vulcanization tube at a rate of speed of between 7.6 to 9.1 meters per minute (25 to 30 feet per minute). This rate of speed was selected to retain the elastomeric material within the steam filled portion of the tube for a long enough period of time to fully cure a protective jacket having an outer diameter of roughly 30 millimeters (one and three sixteenths inches). Of course, the rate of speed at which different sizes of cable can be cured by passing through the same length of vulcanization tube changes with the thickness and heat capacities of the materials to be cured.
A problem with continuous vulcanization processes arises in that the elastomeric material of a protective jacket must be retained within the tube for a period of time which is long enough to heat the entire protective jacket to a cure temperature, and then retained at this cure temperature for a sufficient length of time to fully cure the elastomeric material within the interstices of the insulated conductors. For a specific length of vulcanization tube, the dwell time at which the protective jacket is retained therein determines the speed at which the manufacturing process may be operated. If the dwell time at which the protective jacket is retained within the vulcanization tube could be decreased, in general, the manufacturing process could be operated at a faster rate, and thus improve productivity of the production process.
One way to improve the productivity of the manufacturing process is to batch cure the protective jacket by spooling the cable onto reels, and heating an entire length of cable within an oven. One such example is U.S. Pat. No. 4,675,474, issued on Jun. 23, 1987, and invented by David H. Neuroth, in which an elastomeric, protective jacket was cured after armoring and spooling the cable onto a reel. However, when a protective jacket is batch cured after spooling onto a reel, the insulated conductors therein may not be held in proper position, centered within the protective jacket, but rather may shift to one side. Shifting of insulated conductors within the protective jacket may reduce the wall thickness of elastomeric material about the conductors to a thickness which is insufficient for reliably providing a fluid barrier to prevent wellbore fluid from attacking the insulating material about the conductors.
Another problem with prior art electric cables arises since some electric cables are cured prior to helically wrapping an exterior armor about the protective jacket. Wrapping the exterior armor about a fully cured protective jacket results in applying internal compressive stresses to the jacket material.
Further, in prior art electric cables, the protective jacket is disposed inside of a metal, exterior armor. The elastomeric material forming the protective jacket typically has a higher coefficient of thermal expansion than metal from which the metal, exterior armor is formed. When prior art electric cables are heated to high temperatures found downhole within wellbores, the protective jacket will expand at a greater rate than the exterior armor. The greater rate of thermal expansion of the protective jacket within the armor creates compressive forces which act to apply potentially destructive stresses to the insulation about the electrical conductors.
Some prior art electric cables include longitudinally extending ribs about the exterior surface of the protective jacket. These longitudinally extending ribs provide expansion voids for the elastomeric material of the protective jacket to expand into when heated to wellbore temperatures, and thus aid in reducing thermally induced compressive stresses. When an exterior armor is helically wrapped about this exteriorly ribbed surface after the protective jacket is fully cured, the longitudinally extending ribs are flattened in a helically spiralled pattern. Flattening of ribs during the manufacturing process changes the location of thermal expansion voids, resulting in nonuniform compressive stresses when the electric cable is heated to downhole temperatures within a wellbore.